Methods of direction finding include calculating a fix confidence score for each source of radiation detected by direction finding stations in known separated locations, which stations each produce a set of bearings of the sources each detects, each bearing having an error predetermined for the associated station, and a triangulation process being used to fix the position of the sources. Object location by triangulation is well known in civil applications.
The invention may be applied, for example, to the monitoring of ship movements by coastguards. Radars or radios carried by ships may be distinguishable from one another by reason of the differences in source parameters such as frequency of the radar or radio signal, or the modulation characteristics such as pulse length, pulse repetition frequency or even pulse shape. Thus it may be possible to associate a combination of bearings taken one each from the direction finding stations as being from one particular source with a high degree of confidence on the basis of parameter match alone. Also a combination of bearings may appear to converge on one position but there may be no parameter match, in which event the existence of a source at such a position can be discounted.
However, the sources may match in parameter values and be indistinguishable. This is likely in the case of mass produced shipborne radars. In this event the sets of bearings provided by the direction finding stations need to be analysed on a geometrical basis to establish a confidence level for each apparent convergence of a set of bearings on a source. In practice a parameter match confidence level or score will be combined with a geometrical confidence level, or score, to provide an overall confidence level for any apparent source.